1. Field of the Invention
The present invention relates to a light emitting device, and more particularly to an AlGalnP light emitting diode structure.
2. Description of the Prior Art
The conventional AlGalnP LED has a double heterostructure (DH), as shown in FIG. 8. The LED stacked sequentially, from a bottom thereof, has an n-type ohmic contact electrode 2, a GaAs substrate 3, an n-type (AlxGa1xe2x88x92x)0.5In0.5P lower cladding layer 4 with an Al composition between about 70%-100%, an (AlxGa1xe2x88x92x)0.5In0.5P active layer 5 with an Al composition of 0%-45%, a p-type (AlxGa1xe2x88x92x)0.5In0.5P upper cladding layer 6 with an Al composition 70%-100%, a p-type high energy band gap current spreading layer 7 such as layers of GaP, GaAsP, AlGaAs or GaInP, and a p-type ohmic contact layer 8 as well as a bonding pad 9.
With the composition alternation of the active layer 5, the wavelengths of the light emitted are varied from 650 nm: red to 555 nm: green. A drawback is generally found in the conventional LED, that is: while the light emitted from the active layer 5 towards the substrate 3 will be totally absorbed by GaAs substrate 3. It is because the GaAs substrate has an energy gap smaller than that of the active layer 5. Therefore, the light generated is absorbed resulted in lower light generated efficiency for this kind of conventional AlGaInP LED.
To overcome the substrate 3 light absorption problem, several conventional LED fabrication technologies have been disclosed. However, those conventional technologies still accompany with several disadvantages and limitations. For example, Sugawara et al. disclosed a method published in Appl. Phys. Lett. Vol. 61, 1775(1992), Sugawara et al. inserted a distributed Bragg reflector (DBR) layer in between GaAs substrate and lower cladding layer so as to reflect those light emitted toward the GaAs substrate. However, the reflectivity of DBR layer is usefully only for those light which almost vertically towards the GaAs substrate. With the decrease of injection angle, the reflectivity is drastically decreased. Consequently, the improvement of external quantum efficiency is limited.
Kish et al. disclosed a wafer-bonded transparent-substrate (TS) (AlxGa1xe2x88x92x)0.5In0.5P/GaP light emitting diode, entitled xe2x80x9cVery high efficiency semiconductor wafer-bonded transparent-substrate (AlxGa1xe2x88x92x)0.5In0.5P/GaP light emitting diodesxe2x80x9d on Appl. Phys. Lett. Vol. 64, No. 21, 2839 (1994). The TS AlGaInP LED was fabricated by growing a very thick (about 50 xcexcm) p-type GaP window layer by hydride vapor phase epitaxy (HVPE) formed on epi-layers light emitting structure. Subsequently, the temporary n-type GaAs substrate is selectively removed using conventional chemical etching techniques. After removing the GaAs substrate, the LED epilayer structure is then bonded to an 8-10 mil thick n-type GaP substrate.
For the light illuminated concerned, the TS AlGaInP LED exhibits a two fold improvement in light output compared to absorbing substrate (AS) AlGaInP LEDs. However, the fabrication process of TS AlGaInP LED is very complicated. Since the bonding process is to make two III-V semiconductor wafers directed bond together by heating and pressing for a period of time. Even worse, a non-ohmic contact interface between them is generally found to have high resistance. To manufacture these TS AlGaInP LEDs in high yield and low cost is difficult as a result.
Another conventional technique was proposed by Horng et al., on Appl. Phys. Lett. Vol. 75, No. 20, 3054 (1999) entitled xe2x80x9cAlGaInP light-emitting diodes with mirror substrates fabricated by wafer bonding.xe2x80x9d Horng et al., reported a mirror-substrate (MS) of AlGaInP/metal/SiO2/Si LED fabricated by wafer-fused technology. In LED, AuBe/Au stack layer function as a bonding layer for silicon substrate and epi-layer LED. However, the intensity of the AlGaInP LED is only about 90 mcd under 20 mA injecting current. The light intensity is at least lower than that of TS AlGaInP LED by 40%. It could not be satisfied.
An object of the present invention is thus to provide a LED structure which is composed a newly bonding layer and a transparent substrate.
Firstly, a temporary semiconductor substrate having epi-layers thereon sequentially formed, from a bottom thereof, with an n-type etch stop layer, an n-type cladding layer, an active layer epi-layers, a p-type cladding layer, and a p-type ohmic contact epi-layer is prepared. And then a first metal electrode is formed on the p-type ohmic contact epi-layer.
Thereafter the temporary semiconductor substrate is bonded to a transparent substrate with the p-type ohmic contact epi-layer and the first metal electrode face to the transparent substrate by a BCB, a transparent resin or the like. Next the temporary semiconductor substrate is removed by etching and stopping at the etch stop layer.
After that, two steps of lithographic and etching methods are carried out successively so as to form an opening that exposes the first metal electrode. In the first lithographic and etching step, a trench of about 3 to 6 mils in width is formed, which exposes a portion of the p-type ohmic contact epi-layer. In the second lithographic and etching step, a contact channel of about 0.5 to 3 mils in width is formed to contact the first metal electrode. Thereafter, the processes are performed to form a transparent conductive layer atop the etch stop layer, to form a first boding metal on the contact channel, and a second boding metal (or called second electrode) on the transparent conductive layer.
The second preferred embodiment is modified from the first preferred embodiment. The approaching of forming a trench and a contact channel are as the first preferred embodiment The modified portion is the second boding metal, which is refilled in a preserved hole constructed by photoresist and transparent conductive layer, Thus after the photoresist removal, the second bonding layer is higher than a surface level of the transparent conductive layer.
In the third preferred embodiment, the two step etchings to form a trench and a contact channel are the same as prior two preferred embodiments. Thereafter a contact hole or a recess region is formed in the etch stop layer first, and then a transparent conductive layer is formed on the etch stop layer including refilled the contact hole or the recess region.
In the fourth preferred embodiment, the two step etchings to form a trench and a contact channel are the same as before, A dielectric region is then formed in the etch stop layer. Thereafter the processes of forming the transparent conductive layer and two bonding metals are as the first prior embodiment.
In the fifth preferred embodiment, the processes are modified from the fourth preferred embodiment. Instead of forming a dielectric region, a high resistance region is formed in the etch stop layer by ion implant with nonconductive ions.